DESCRIPTION (Investigator's Abstract): Natural modifications in nucleic acids, such as those which occur in RNA following transcription, are implicated in a variety of roles in cellular processes, including stabilization of secondary and tertiary structure, modulation of protein- nucleic acid interactions, and adaptation to cellular stress. It is proposed to develop new and improved analytical strategies for structural characterization and sequence placement of modified nucleotides in nucleic acids, based primarily on electrospray ionization and tandem mass spectrometry. Emphasis will be placed on E. coli 16S rRNA as a model system, but elements of the methods proposed are also applicable to structural problems involving DNA, xenobiotically modified nucleic acids and synthetic oligonucleotide analogs.